The present invention relates to a semiconductor memory, and in particular to a circuit for improving the access time of a semiconductor memory.
A semiconductor memory, such as a bipolar memory, comprises a number of memory cells arranged in the form of matrix. Each cell is connected between a pair of word lines arranged in a row and a pair of digit lines arranged in a column.
When one of the memory cells is selected for writing or reading, an addressing pulse is applied to one of the word lines, which is connected to the selected cell.
The access time of this kind of memory is determined mainly by both the rising and falling time of word line potential changing in response to the addressing pulse applied thereto.
As far as the rising time of the potential on the word line is concerned, the prior art provides some successful solutions to realize a quick or abrupt rising of the potential.
However, the improvement on the falling time of the potential on the word line was considered much more difficult to achieve. In the case where a large number of memory cells are used for a memory array, it is necessary to consider the adverse effect of stray capacity between the word line and ground on the falling time of the potential on the word line. When one of the memory cells is selected for reading or writing, the word line connected thereto is switched to a high potential. This results in the stray capacity between the word line and ground being charged up. Upon the completion of a reading or writing cycle, the word line is returned to a low potential. But, due to the charge stored in the stray capcity during the reading or writing cycle, it takes a relatively long time to clamp the word line to a low potential. This slow falling of the potential on the word line prevents the memory from being operated at a high speed.
One conventional and insufficient attempt to solve this problem is to provide an additional current to the selected cell during the time that the word addressing pulse is being applied to the word line. Measurements showed, however, that the improvement by this attempt was relatively small.